git-arc: fix diff2reviewers() with multiple reviewers

The select command of jq provides us with a multiline output.  Thus, the
${userids} is a newline separated list.  The recent change 9e84289f2c
explicitly said that we aren't going to translate newline to comma when
constructing JSON for the next command.  It also forces the ${userids} to
be printed as is, with shell's field splitting disabled.  So we end up
with invalid JSON, that is missing commas.

Fix this by enabling shell's field splitting by printing ${userids}
outside of double quotes.

Reviewed by:		jlduran, imp, emaste
Differential Revision:	https://reviews.freebsd.org/D47889
Fixes:			9e84289f2c

tools/tools/git/git-arc.sh

@@ -341,7 +341,7 @@ diff2reviewers()
         jq '.response.data[0].attachments.reviewers.reviewers[] | select(.status == "accepted").reviewerPHID')
     if [ -n "$userids" ]; then
         echo '{
-        "constraints": {"phids": ['"$(echo "$userids" | tr '[:blank:]' ',')"']}
+        "constraints": {"phids": ['"$(echo $userids | tr '[:blank:]' ',')"']}
         }' |
         arc_call_conduit -- user.search |
         jq -r '.response.data[].fields.username'